1. Field of the Invention
This invention relates generally to the field of urethane prepolymer formation and, more specifically, this invention relates to a method of forming a urethane prepolymer which exhibits improved capacity for binding aggregate material such as peat and for forming high quality foams.
2. Description of the Prior Art
The use of various polymeric materials to provide support for aggregate material, such as the use of polymeric foam to form a matrix for plant growth-supporting nutrients, for example, is well known.
One form of polymeric matrix which has found wide acceptance in the soil consolidation art is polyurethane form which comprises a polyether or polyester urethane formed by the process conventionally known as the "one-shot" process, or from a liquid urethane prepolymer which is reacted with an aqueous slurry of aggregate material.
In one form of the slurry reaction referred to above, a hydrophilic liquid urethane prepolymer which comprises an isocyanate-capped polyol with an excess of free isocyanate groups is reacted with a slurry comprising aggregate material, water, and any of various modifying agents such as blowing agents, promoters, accelerators, or other modifying ingredients.
The following U.S. patents, the disclosures of which are hereby incorporated by reference, provide examples of prior approaches to aggregate material consolidation with a urethane foam matrix.
Pruitt et al. U.S. Pat. No. 3,373,009 discloses a method of forming a nutrient plant growth medium from a urethane made by reacting a polyether of polyester compound and a diisocyanate at a temperature below 120.degree. C. for a period of between about 2 and 4 hours. The prepolymer is then mixed with an aqueous mixture of catalyst, plant nutrient, and selected modifying ingredients to produce a foam polyurethane polymer intimately incorporating the plant nutrients.
Kristner U.S. Pat. Nos. 3,805,531 and 3,805,532 describe a reaction scheme wherein a urethane prepolymer is formed by mixing a compound having a plurality of active hydrogen atoms, such as a polyol, with a stoichiometric excess of an organic polyisocyanate or polyisothiocyanate compound at ambient temperature under essentially anhydrous conditions for a period of several days.
Wood et al. U.S. Pat. Nos. 3,812,619 and 4,241,537 describe a similar reaction conducted at a temperature below 120.degree. C. for a period of about 6 hours.
All known prior methods of forming a urethane prepolymer ultimately used to produce a plant growth medium involve a maximum reaction temperature of 120.degree. C., and generally require rather lengthy reaction and/or aging (curing) periods. This maximum temperature limitation has been thought necessary due to the strongly exothermic nature of the isocyanate-polyol capping reaction, and the common belief that undesirable spurious side reactions, such as homopolymerization of isocyanate, are encountered to an unacceptable degree at elevated temperatures above 120.degree. C. The presumed existence of these side reactions even at temperatures below 120.degree. C. has in the past resulted in the use of relatively pure reagents.